1. Field of the Invention
The present invention relates to a fluid separation element assembly, and more specifically to a fluid separation element assembly which is suitable for use in an apparatus for reverse osmosis, ultrafiltration or microfiltration.
2. Description of the Related Art
Generally, a spiral type fluid separation element assembly is formed, for example, as shown in FIG. 20. In FIG. 20, a permeate carrier material 156 is disposed between a first separation membrane 154 and a second separation membrane 155, the first separation membrane 154 and second separation membrane 155 are closed at three sides by, for example, an adhesive, and the remaining one side is opened in a direction toward a permeate collection tube 153 having permeate collection openings 152 to form an envelope member. A membrane unit 160 including this envelope member and a feed carrier material 157 is spirally wound around the permeate collection tube 153. An anti-telescoping plate 158 is attached on the downstream-side end surface of the wound membrane unit 160, a seal holder 159 is attached on the upstream-side end surface thereof, and a wrapping material 161 is formed on the periphery of the membrane unit 160. As the seal holder 159, generally the same plate as the anti-telescoping plate 158 is used.
In such a fluid separation element assembly 151, feed water having entered from the side of seal holder 159 through a feed water passage 162 is separated to water 164a permeated through separation membranes 154 and 155 and the remaining feed water 164b during being passed through feed carrier material 157. The permeated water 164a is discharged from the exit of permeate collection tube 153, and the remaining feed water 164b (concentrated water) is discharged from the exit of anti-telescoping plate 158, respectively. Anti-telescoping plate 158 prevents a telescoping deformation of fluid separation element 151 due to a pressure loss caused when feed water 163 passes through feed carrier material 157. Anti-telescoping plate 158 and seal holder 159 are integrally combined with membrane unit 160 so that they are not easily harmed by the load originating from the pressure loss. Therefore, when fluid separation element 151 has deteriorated to a condition that it cannot further be used, for example, because of reduction of separation performance of the separation membrane, the members such as anti-telescoping plate 158 and permeate collection tube 153 are discarded without being reused, even if they are still usable.
On the other hand, a fluid separation element assembly having a detachable anti-telescoping plate is disclosed in U.S. Pat. No. 4,906,372. In the fluid separation element assembly, as shown in FIG. 21, because there is no means for preventing feed water from leaking toward the periphery side of a fluid separation element 151 from a gap between the fluid separation element 151 and an anti-telescoping plate 158 attached on the upstream-side end surface of the fluid separation element 151, a net member 165 is provided on the periphery of the fluid separation element 151.
In the fluid separation using such a conventional fluid separation element assembly, as shown in FIG. 22, fluid separation element 151 is contained in a pressure vessel 166 to be used as a fluid separation membrane module 167. Net member 165 attached on the periphery of fluid separation element 151 is also contained, and the gap between pressure vessel 166 and fluid separation element 151 may be sealed. In this case, although net member 165 has elasticity, in order that the net member 165 is shrunk when it is inserted into pressure vessel 166, it is necessary to carefully finish the accuracy of the diameter of fluid separation element 151 and the thickness of the net member 165, depending upon the variable accuracy of the inner diameter of the pressure vessel 166.
In practice, however, fluid separation element 151 undergoes a dispersion in its diameter more or less, depending upon a dispersion of the thickness of the member(s) used for the fluid separation element 151. Because relatively many different kinds of members are used for fluid separation element 151 and a relatively large amount of them is used therefor, for example, in an 8 inch fluid separation element, there occurs a dispersion in the thickness of the diameter of the fluid separation element of 2 to 3 mm. When the diameter of fluid separation element 151 including net member 165 is smaller than the inner diameter of pressure vessel 166, the net member 165 does not comes into contact with the pressure vessel 166, and because the gap between the pressure vessel 166 and the fluid separation element 151 cannot be sealed, a relatively large amount of feed water 163 passes through the gap between the pressure vessel 166 and the fluid separation element 151. As a result, the amount of feed water 163 passing along the surface of the separation membrane decreases, the concentration polarization on the surfaces of separation membranes 154 and 155 becomes great, and the amount of permeated water 164a decreases as well as the salt rejection performance in the membrane separation is greatly reduced. Particularly, in a process such as desalting of sea water in which the feed water has a high salt concentration, the affection due to the concentration polarization is great, the quality of the permeated water 164a is remarkably reduced.
It may be possible to increase the property of shrinkage of net member 165 by making the diameter of the portion of membrane unit 160 fairly smaller than the inner diameter of pressure vessel 166 in anticipation of the dispersion of the diameter of fluid separation element 151, and by making the net member 165 thick. In this case, however, because it is necessary to make the portion of membrane unit 160 of fluid separation element 151 small, the effective membrane area becomes small, and the amount of water treated by the fluid separation element 151 is decreased.